The vertebrate adaptive immune system is defined by the repertoire of immunoglobulin (Ig) and T-cell receptors (TCR) found in its genome. Although the proteins encoded by the Ig and TCR genes play different roles in host defense, their genomic organization has led to the hypothesis that they have evolved from a common progenitor. The novel immune-type receptor (NITR) genes to be studied in this proposal are phylogenetically distinct but unmistakenly related to Ig and TCR genes and may be related to and exhibit characteristics of the ancestral gene. This relationship to Ig/TCR genes and the observation that NITR genes are expressed in hematopoeitic tissues suggests that they are involved in the vertebrate immune response. Established molecular biology techniques will be used to characterize the genomic organization and expression patterns of the NITR genes in the zebrafish, Danio rerio. A comparison of expression patterns of the NITR genes to expression of the Ig and TCR genes in normal and mutant animals will help define their cell lineage and yield insight into the functional role of these genes. Sequencing and genomic mapping of the zebrafish NITR genes will aid in the identification of homologous genes in other species, including mammals.